The present invention relates broadly to a method of reinforcing a sheet-like substrate to provide a substrate with improved tear resistance and tensile strength in both the in-machine and cross-machine directions. More specifically, the present invention is the process and product formed by the process of applying a plurality of linear reinforcing members comprising an essentially filamentary core pre-coated with a hardened thermoplastic adhesive to a sheetlike substrate web in a pattern wherein at least one of the linear reinforcing members has both in-machine and cross-machine directional components.
Linear reinforcements that include a filamentary core pre-coated with a thermoplastic "hot melt" adhesive are known in the prior art. The assignee of the present invention, H. B. Fuller Company, has developed and is marketing such reinforcements under the trade names LINEAR.RTM. Adhesives and STRING KING.RTM.. The STRING KING.RTM. product typically has a core of glass, polyester, cotton, polypropylene or other fibers and the "hot melt" adhesive has a softening point at temperatures elevated somewhat above the ambient and generally has an ethylene-vinyl acetate/wax/tackifying resin formula of a type known in the art. These reinforcing products may be applied and have been applied by passing the linear reinforcing members onto a continuous web and heating the "hot melt" adhesive above its softening point. A plurality of parallel lines of reinforcements has been applied between a linear and a fluted web in the manufacture of reinforced corrugated board. The pre-coated adhesive is then elevated above its softening point and allowed subsequently to harden. The linear reinforcements thus applied provide significant tensile strength in what can be defined as the in-machine direction of the moving web and tear resistance at angles to the in-machine direction. However, such linear reinforcements provide little or no tensile strength in the transverse or cross-machine direction and/or tear resistance in the in-machine direction.
It is also known in the prior art to reinforce a laminated structure by incorporating string or thread-like reinforcement between the laminations to provide cross-machine and in-machine strength. Such methods include relatively complex systems for the application of an adhesive to bond the laminations independently of the string reinforcements. In the known processes, the string reinforcements are drawn through a liquid bath of adhesive and then allowed to harden bonding the laminations with the string or thread-like reinforcements therebetween.
The present invention is a method substantially less complex, more flexible, and more economical than the prior art processes for providing in-machine and cross-machine reinforcements to a sheet-like substrate and/or lamination. The process of the present invention utilizes the advantages of reinforcements pre-coated with "hot melt" type adhesives which have rapid setting features. An infinite number of reinforcement patterns can be applied to the sheet-like substrate in the method of the present invention.
Utilizing the method of the present invention, an improved reinforced sheet-like substrate, and in particular an improved paper substrate, can be manufactured having the linear reinforcements and pre-coated "hot melt" adhesive applied to the surface thereof in a generally serpentine pattern achieving substantial tear resistance and tensile strength in the in-machine and cross-machine directions to eliminate the need for multi-ply bags. Additionally, the reinforcement applied in the method of the present invention may lead to the use of lower basis weight or less costly paper on single ply bags and the production of bags with sufficient strength to package wet products without the need for wet strength paper.